Steel material composition for producing piston rings and cylinder liners

ABSTRACT

A steel material composition, in particular for producing piston rings and cylinder liners, comprises the following elements in the listed quantities, relative to 100% by weight of the steel material: 0.5-1.2% by weight C, 0-3.0% by weight Cr, 72.0-94-5% by weight Fe, 3.0-15.0% by weight Mn and 2.0-10.0% by weight Si. The composition can be produced by producing a melt of the starting materials and casting the melt in a prefabricated mold.

The present invention relates to a steel material composition that issuitable particularly for producing piston rings and cylinder sleeves.The present invention further relates to a method for manufacturing thesteel material composition according to the invention. Finally, thepresent invention relates to piston rings and cylinder sleeves thatcomprise the steel material compositions as the basic elements thereof.

RELATED ART

Piston rings in a combustion engine seal the gap that exists between thepiston head and the cylinder wall off from the combustion chamber. Asthe piston moves up and down, the outer peripheral surface of the pistonring slides along the cylinder wall in permanently spring-biased contacttherewith, while the piston ring itself oscillates as it travels in itspiston ring groove due to the tilting movements of the piston, and thisoscillation causes the flanks of the ring come into contactalternatingly with the upper and lower flanks of the piston ring groove.As the two elements slide over one another, each is subject to a certainamount of wear depending on the nature of the material, and in the eventof dry running this can lead to seizing, scoring and ultimately causeirreparable damage to the engine. In order to improve the sliding andwearing behaviour of piston rings with respect to the cylinder wall, theperipheral surfaces of the piston rings have been coated with variousmaterials.

In the case of cylinder sleeves such as those used in reciprocatingpiston internal combustion engines, a high degree of wear resistancemust be assured, otherwise, that is to say as the cylinder sleevebecomes thinner, gas leakage and oil consumption may increase and theperformance of the engine will deteriorate. As the cylinder sleevebecomes worn, the gap between the cylinder wall and the cylinder sleevebecomes steadily larger, with the result that combustion gases are ableto escape past the cylinder sleeve more easily (referred to as“blow-by”), which in turn reduces engine efficiency. Because of theenlarged gap, the oil film that is not stripped off and is left in thecombustion chamber becomes thicker, with the result that more oil can belost per unit of time, effectively increasing oil consumption.

The parts of internal combustion engines that are exposed to highstresses, such as piston rings and cylinder sleeves, are usually madefrom cast iron materials or cast iron alloys. Piston rings andparticularly compression rings in high-performance engines are exposedto increasing stresses, including peak compression pressure, combustiontemperature, EGR and lubrication film reduction among others, and whichhave a critical effect on their functional properties, such as wearscuff resistance, microwelding and corrosion resistance.

Unfortunately, cast iron materials according to the prior art are highlysusceptible to breakage, and rings often break when the existingmaterials are used. Higher mechanical-dynamic loads result in shorteroperating lives for piston rings and cylinder sleeves. Running surfacesand flanks are subject to heavy wear for the same reasons.

Higher ignition pressures, reduced emissions and direct fuel injectioncontribute to increased loads on the piston rings. As a result, thepiston material is damaged and deposits accumulate on it, particularlyon the lower piston ring flank.

Having to deal with higher mechanical and dynamic loads on piston ringsand cylinder sleeves, more and more engine manufacturers are requestingpiston rings and cylinder sleeves that are made from high-grade steel(annealed and high-alloyed, such as the material 1.4112). In thiscontext, iron materials containing less than 2.08% by weight carbon areclassified as steel. If the carbon content is higher, the material isconsidered to be cast iron. Steel materials have better strengthproperties and ductile values than cast iron because theirmicrostructures are not disrupted by free graphite.

The steels used most frequently to produce steel piston rings orcylinder sleeves are high chrome alloy, martensitic steels. Steel pistonrings are manufactured from profile wire. The profile wire isroundwound, cut to length and drawn over an “out-of-round” mandrel. Onthis mandrel, the piston ring is given its desired out-of-round shape inan annealing process, which also sets up the requisite tangentialforces. A further disadvantage of manufacturing piston rings from steelis that above a certain diameter, it is no longer possible to produce(wind) rings from steel wire. In contrast, cast iron piston rings arealready cast out of round, so they are ideally shaped from the outset.

Cast iron has a considerably lower melting temperature than steel. Thedifference may be as much as 350° C. depending on chemical composition.Cast iron is therefore easier to melt and cast, since a lower meltingtemperature means a lower casting temperature and thus also lessshrinkage due to cooling, so that the case material has few blowholesand/or hot or cold cracks. A lower casting temperature also generatesless stress on the moulding material (erosion, gas porosities, sandinclusions) and the furnace as well as lower melting costs.

The melting temperature of the iron material depends not only on itscarbon content but also on the “degree of saturation”. The followingformula, shown in simplified form, applies:

S_(c)=C/(4.26−l/3 (Si+P))

The closer the degree of saturation is to 1, the lower the meltingtemperature is. In the case of cast iron, a degree of saturation of 1.0is usually aimed for, wherein the cast iron has a melting temperature of1150° C. The degree of saturation of steel is about 0.18 depending onits chemical composition. Eutectic steel has a melting temperature of1500° C.

The degree of saturation can be influenced considerably by the Si or Pcontent. For example a 3% by weight increase in silicon content has asimilar effect to a 1% by weight increase in C content. It is thuspossible to produce a steel material having a C content of 1% by weightand 9.78% by weight silicon that has the same melting temperature ascast iron with a degree of saturation of 1.0 (C: 3.26% by weight, Si:3.0% by weight).

If the Si content is increased significantly, the degree of saturationof the steel material may also be increased and the melting temperaturelowered to the same level as cast iron. In this way, it is possible toproduce steel using the same equipment as is used to produce cast iron,for example GOE 44.

Piston rings and cylinder sleeves made from a steel casting materialwith high silicon content are known in the prior art. However, thepresence of a larger quantity of silicon has a negative effect on thehardenability of the material, because its “Ac3” austenitic conversiontemperature is raised.

BRIEF DESCRIPTION OF THE INVENTION

In view of the above, the object of the present invention is to providea steel material composition with high silicon content, particularly forproducing piston rings and cylinder sleeves, having improvedhardenability. Due to its production in a gravity casting process, thesteel material composition should improve on the properties of annealedcast iron with spheroidal graphite with respect to at least one of thefollowing parameters:

-   -   Mechanical properties such as e-modulus, bending strength    -   Resistance to breakage    -   Mechanical stability    -   Flank wear    -   Running surface wear

This object is solved according to the invention with a steel materialcomposition including the following elements in the proportionsindicated:

-   C: 0.5-1.2% by weight-   Cr: 0 -3.0% by weight-   Fe: 72.0-94.5% by weight-   Mn: 3.0-15.0% by weight-   Si: 2.0-10.0% by weight

The content substances are contained in such manner that the sum of allstarting materials, components, content substances, elements andadditives, whether indicated specifically or not explicitly named, isequal to 100% in all cases. The proportion of starting materials,components, content substances, elements and additives may be adjustedby various methods known to one skilled in the art. The chemicalcomposition is adjusted with particular reference to the workpiece to beproduced.

The manganese contained functions as an austenite former that extendsthe gamma range and shifts the Ac3 austenitic conversion temperatureupwards. In this way, improved hardenability of the steel material isachieved according to the invention.

The following elements are contained in the steel material compositionaccording to the invention preferably in proportions not exceeding thevalues indicated relative to 100% by weight of the steel materialcomposition:

Al: max. 0.02% by weight P: max. 0.1% by weight B: max. 0.1% by weightS: max. 0.05% by weight Cu: max. 2.0% by weight Sn: max 0.05% by weightMo: max. 3.0% by weight Ti: max. 1.5% by weight Nb: max. 0.05% by weightV: max. 1.5% by weight Ni: max. 4.0% by weight W: max. 1.5% by weightwherein the sum of the fractions of Nb, Ti, V and W does not exceed 1.5%by weight.

It is further preferred according to the invention that the steelmaterial composition according to the invention contains only elementsselected from the group consisting of Al, B, C, Cr, Cu, Fe, Mn, Mo, Nb,Ni, P, S, Si, Sn, Ti, V and W, and that the sum of these elements isequal to 100% by weight.

The steel material composition according to the invention reduces thesusceptibility of workpieces made therefrom to become deformed in thepresence of extreme heat, thus ensuring high performance capability forthe long term and also reducing oil consumption. Because of itsexcellent properties, the steel material composition according to theinvention is therefore ideally suitable for the production of pistonrings and cylinder sleeves in the automotive and LB fields, or for valveseat inserts and guides. Additionally, drive seals, carrier plates forbrake linings on disc brakes (black plates) and rings for cooling units,pump nozzles and cylinder sleeves (liners) as well as shaft sleeves andparts for the chemical industry may be manufactured.

The steel material composition according to the invention also has theadvantage that it thus becomes possible to manufacture steel pistonrings and cylinder sleeves, for example, using the machinery andtechnologies that are normally used for manufacturing cast ironworkpieces. Moreover, the production costs are equivalent to those forcast iron piston rings, affording the manufacturer a cost advantage andimproved value creation. The material parameters are also adjustableindependently of the supplier.

A method for producing a steel material composition according to theinvention is also provided according to the invention, which methodincludes the following steps:

-   -   a. Producing a melt from the starter materials, and    -   b. Pouring the melt into a prefabricated mould.

Steel scrap, recycled material and alloys, for example, may be used asstarter materials. The smelting process takes place in a furnace,preferably a cupola furnace. Following this, the melt is allowed tosolidify to produce a blank. The blank may be cast using methods knownin the related art, such as centrifugal casting, continuous casting,punch pressing methods, Croning, or preferably green sand moulding.

After the steel material composition has cooled, the form is emptied andthe blank obtained is cleaned.

If necessary, the blank may then be annealed. This is done in thefollowing steps:

-   -   c. Austenitising the steel material composition above its Ac3        temperature,    -   d. Quenching the steel material composition in a suitable        quenching medium, and    -   e. Tempering the steel material composition at a temperature in        the range from 400 to 700° C. in a controlled atmosphere        furnace.

Oil is preferably used as the quenching medium.

To harden the steel material composition according to the inventionfurther, the steel material composition obtained thereby may be nitridedfollowing the process steps described in the preceding. This may beperformed for example by gas nitriding, plasma nitriding or pressurenitriding.

The following example explains the invention without being limitedthereto.

Example

A piston ring was produced from a steel material composition accordingto the invention having the following composition:

Al: 0.002% by weight P: 0.03% by weight B: 0.1% by weight S: 0.009% byweight C: 0.7% by weight Si: 3.0% by weight Cr: 2.0% by weight Sn:0.001% by weight Cu: 0.05% by weight Ti: 0.007% by weight Mn: 5.05% byweight V: 0.015% by weight Mo: 0.5% by weight W: 0.011% by weight Nb:0.002% by weight Fe: Remainder

This was done by producing a melt of the starter materials (steel scrap,recycled material and alloys), and pouring the melt into a prefabricatedgreen sand mould. Then, the mould was emptied and the piston ring thusobtained was cleaned. The piston ring was then annealed. This isachieved by austenitising above the Ac3 temperature of the steelmaterial composition, quenching in oil, and tempering in a controlledatmosphere furnace at a temperature in the range from 400 to 700° C.

1. A steel material composition, particularly for producing piston ringsand cylinder sleeves, characterized in that it contains the followingelements in the proportions indicated relative to 100% by weight of thesteel material composition: C: 0.5-1.2% by weight Cr: 0-3.0% by weightFe: 72.0-94.5% by weight Mn: 3.0-15.0% by weight Si: 2.0-10.0% by weight2. The steel material composition as recited in claim 1, characterizedin that it also contains the following elements in quantities notexceeding the proportions indicated: Al: max. 0.02% by weight B: max.0.1% by weight Cu: max. 2.0% by weight Mo: max. 3.0% by weight Nb: max.0.05% by weight Ni: max. 4.0% by weight P: max. 0.1% by weight S: max.0.05% by weight Sn: max 0.05% by weight Ti: max. 1.5% by weight V: max.1.5% by weight W: max. 1.5% by weight wherein the sum of the fractionsof Nb, Ti, V and W does not exceed 1.5% by weight.
 3. The steel materialcomposition as recited in claim 1 or 2, characterized in that it onlycontains elements selected from the group consisting of Al, B, C, Cr,Cu, Fe, Mn, Mo, Nb, Ni, P, S, Si, Sn, Ti, V and W, and wherein the sumof these elements is equal to 100% by weight.
 4. A method for producinga steel material composition as recited in any of claims 1 to 3,including the following steps: a. Producing a melt from the startermaterials, and b. Pouring the melt into a prefabricated mould, and thefollowing steps as necessary: c. Austenitising the steel materialcomposition above its Ac3 temperature, d. Quenching the steel materialcomposition in a suitable quenching medium, and e. Tempering the steelmaterial composition at a temperature in the range from 400 to 700° C.in a controlled atmosphere furnace.
 5. A method for producing a steelmaterial composition as recited in claim 4, further including thefollowing step: f. Nitriding the steel material composition obtained. 6.A piston ring, characterized in that it comprises a steel materialcomposition as recited in any of claims 1 to 3 as the basic elementthereof.
 7. A cylinder sleeve, characterized in that it comprises asteel material composition as recited in any of claims 1 to 3 as thebasic element thereof.